This invention relates to control of phase shift for a voltage controlled oscillator.
A conventional approach of an LC-based ring oscillator uses three LC stages, with a phase shift sum for the three stages being 180xc2x0, as required to support an oscillation. Use of three stages is necessary with a conventional approach, because any stage provides a phase shift of less than 90xc2x0, except at certain extreme or unrealistic choices of parameter values. In the simplest three-stage oscillator, each stage provides 60xc2x0 of phase shift. This arrangement is not suitable for applications that require in-phase and quadrature clock signals that are spaced 90xc2x0 apart.
Some workers have attempted to handle this problem by providing two couplers and two identical LC-based voltage controlled oscillators (VCOs) to form a coupling loop. The couplers modulate the phases of the LC tank circuits and force the two clock signals to run in quadrature, with a relative phase shift of 90xc2x0. However, this arrangement operates at a frequency that is relatively far from the peak LC tank impedance so that the oscillator is less efficient than desired. This arrangement becomes less attractive as the clock frequency increases, because the couplers increase the capacitive load of the oscillator. This lowers the central frequency and/or reduces the frequency tuning range.
What is needed is a configuration that improves the efficiency of the, oscillator and increases the center frequency and/or frequency tuning range.
The invention meets these needs by incorporating a selected signal buffer in each stage of the coupling loop so that the buffer and an associated VCO maintain 90xc2x0 phase shift within each stage. The additional phase shift introduced by each buffer, helps move the operating point of the oscillator toward the peak impedance point of the LC tank circuit, thus improving the efficiency of the oscillator. The capacitive loading of each oscillator stage is reduced, because the coupler in each stage is only required to drive a signal buffer, not a combination of buffer and in-path coupler. This provides an increase in center frequency or frequency tuning range.